This application claims priority under 35 U.S.C. xc2xa7xc2xa7119 and/or 365 to 9800275-1 filed in Sweden on Jan. 30, 1998; the entire content of which is hereby incorporated by reference.
The invention relates generally to encrypted connections and more specifically to a method and an arrangement for establishing an encrypted connection between a mobile station and a base station in a mobile telephone system.
During the establishment phase of a GSM call, the network operator may optionally choose to start encryption of the call by transmitting a cipher mode command (CMC) message [cp. GSM 04.08 Mobile Radio Interface Layer 3 Specification] to the mobile station (MS) to order it to encrypt the call.
If the CMC message is transmitted on the radio interface to the MS under bad radio conditions, the consequence may be that the call establishment attempt fails. Even worse, the radio channel may also be blocked for further use during some time.
Transmission of information frames (I-frames) on radio interface is handled within the Link Access Procedures on the Dm channel (LAPDm) protocol [cp. GSM 04.06 MS-BSS Interface Data Link Layer Specification]. When an I-frame is lost due to bad radio environment, the situation is solved by retransmitting the I-frame. This is a general method that is used for all I-frames with no respect to what message the I-frame contains.
Unlike all other messages sent to the MS, the CMC message is special in that it also controls a function in a base transceiver station (BTS), namely encryption.
The changing of cipher mode in the BTS is done in two steps. First, the cipher mode is changed in the uplink direction, i.e. the direction from the MS to the network.
When the uplink operates using the new cipher mode, the cipher mode is also changed in the downlink direction, i.e. the direction from the network to the MS.
The CMC message orders the MS to use the new cipher setting in both directions. At the same time as the CMC message is sent on the radio interface, the BTS starts deciphering the uplink according to the new cipher mode setting. When the BTS notices that the uplink is enciphered according to the new cipher mode setting, the BTS also starts enciphering the downlink.
A main-signalling-link, i.e. a signalling link within the LAPDm protocol that is used for controlling the MS, is established between the BTS and the MS by the MS by sending a set asynchronous balanced mode (SABM) frame to the BTS. The BTS acknowledges the reception of the SABM frame by sending an unnumbered acknowledge (UA) frame to the MS. When the MS receives the UA frame, it enters the multiple-frame-mode, i.e. the acknowledged operation mode of a LAPDm link where information exchange must be acknowledged by the receiver of the information. If the UA frame was not correctly received by the MS, the MS remains in idle-mode, i.e. the unacknowledged operation mode of a LAPDm link where information exchange need not to be acknowledged by the receiver of the information.
It is absolutely necessary that the MS is in multiple-frame-mode when the CMC message is received. Otherwise, it will be ignored by the MS.
When the BTS has sent the CMC message to the MS, the BTS sets the new cipher mode in uplink and assumes that the MS will use the new cipher mode setting in both directions. However, if the MS is in idle-mode, it will not accept the message and consequently not encrypt the uplink with the requested cipher mode.
Thus, a situation may arise where the BTS decrypts the uplink according to the new cipher mode setting, while the MS continues to use the old cipher mode setting or continues to be in unciphered mode. In this situation, the BTS will not be able to understand the messages that the MS sends uplink.
The BTS interprets the situation that the MS does not acknowledge the I-frame as it should, according to the LAPDm protocol. In this situation, the BTS will repeat the I-frame, which will be ignored by the MS, as it is in idle-mode. When the BTS has repeated the I-frame a predetermined number of times, an error report indicating LAPDm problem will be sent to a base station controller (BSC). The BSC action on this is to release the radio channel.
The MS will repeat the SABM frame. The SABM frame is not encrypted by the MS, but the BTS decrypts it according to the new cipher mode. The BTS will not understand the SABM frame, and consequently not respond to it. When the MS has repeated the SABM frame a predetermined number of times without receiving any response, it will stop trying to establish the link.
Today, there is no way of avoiding the problem of losing the call and blocking the radio channel due to mismatch between the cipher settings in the BTS and MS, respectively.
The object of the invention is to eliminate the problems described above.
This is attained in that the BTS makes sure, before sending a CMC message to an MS, that the MS considers the main-signalling-link to be in multiple-frame-mode.
Hereby, no calls will be lost and the radio channel will not be blocked for the reason mentioned above.